An adaptive modulation and coding rate communication system conventionally used changes a cording rate of error correction code and a degree of multi-value modulation according to propagation path quality. Specifically, this system provides high-speed data communication for a user with a good propagation path quality while sacrificing noise-resisting characteristics, and provides low-speed data communication for a user with a poor propagation path quality, attaching importance to noise-resisting characteristics.
The communication system using such an adaptive modulation is used in radio communication systems such as GSM EDGE, HDR and the like. Also, a similar system is expected to be used additionally in W-CDMA.
An example of the adaptive modulation and coding rate communication system will be described with reference to Table 1 below.
TABLE 1ModeCoding methodModulating method0R = 1/2QPSK1R = 1/216-QAM2R = 3/416-QAM3R = 3/464-QAM
First, reception quality data indicating quality of received data is transmitted from a receiver to a base station. At the base station, the reception quality data is classified into four grades (modes 0 to 3). The mode 0 indicates the lowest quality, and the mode 3 indicates the highest quality. The base station selects a coding method and a modulating method as shown in Table 1 on the basis of the mode of the reception quality data.
Table 1 shows two coding methods represented by R=½ and R=¾. The coding method represented by R=½ adds one redundant bit to one bit of input data. The coding method represented by R=¾ adds one redundant bit to three bits of input data. Modulating methods include known QPSK, 16-QAM, and 64-QAM.
A relation of amounts of data transferred is expressed as follows:(R=½, QPSK)<(R=½, 16-QAM)<(R=¾, 16-QAM)<(R=¾, 64-QAM)
On the other hand, a relation of noise resisting characteristics is expressed as follows:(R=½, QPSK)>(R=½, 16-QAM)>(R=¾, 16-QAM)>(R=¾, 64QAM)
Thus, the amounts of data transferred and the noise resisting characteristics have relations opposite to each other.
Hence, when a coding method and a modulating method are selected as shown in Table 1, the mode 0, indicating the lowest quality, allows communications with enhanced noise resisting characteristics. The mode 3, indicating the highest quality, allows communications transferring a large amount of data.
With the conventional technology as described above, reception quality data is transmitted from a receiver to a base station, and the base station selects a combination of an optimum modulating method and coding method on the basis of the reception quality data. Thus, the receiver is required to maintain good reception characteristics in all combinations of the modulating methods and coding methods handled by the base station.
A comparison of the modulating methods QPSK, 16-QAM, and 64-QAM indicates that 64-QAM is more susceptible to a shift in reception timing, an error in synchronous detection and the like, as compared with the other methods. FIG. 1 is a graph showing a comparison of effects of shifts in reception timing on the modulating methods. As shown in FIG. 1, 64-QAM is most susceptible to shifts in reception timing. FIG. 2 is a graph showing a comparison of effects of errors in synchronous detection on the modulating methods. As shown in FIG. 2, 64-QAM is most susceptible to shifts in reception timing.
64-QAM and 16-QAM are susceptible to multipath interference specific to a mobile communication environment. Accordingly, when 64-QAM or 16-QAM is used, it is necessary, for efficient communication, to suppress the interference by using an interference canceller and an equalizer.
Thus, a high-performance receiver compatible with the adaptive modulation and coding rate communication system needs to be designed in accordance with the mode (64-QAM in this case) in which accuracy of each receiving function is most likely to deteriorate due to a shift in reception timing and the like.
Such high performance, however, is not necessary at the time of reception in a mode in which accuracy of the receiving function does not deteriorate.
Also, highly accurate reception processing generally requires an increase in speed of signal processing, and thus increases power consumption. A mobile terminal such as a portable telephone, of which low power consumption is required, needs to perform only a minimum function required for reception. Hence, there is a problem with operation at all times of a functional block designed for the 64-QAM mode.
It is accordingly an object of the present invention to provide a communication apparatus and the like that eliminate the need for performing at all times the function provided for the mode in which reception accuracy tends to deteriorate in the adaptive modulation communication.